Japanese Patent Application Publication (JP-B) No. H6-19147 discloses a column base structure. In this column base structure, a base plate is joined to anchor bolts that are buried in a concrete foundation. A column is joined to the base plate, and the center of a projected outline of the column with respect to the horizontal plane is offset with respect to the center of a projected outline of the base plate with respect to the horizontal plane. In this column base structure, an out-of-plane deformation prevention means is provided to the base plate on the opposite side to the side to which the column is offset. The out-of-plane deformation prevention means is configured by washers, anchor bolts, or ribs. When bending moment arises in the column, the out-of-plane deformation prevention means prevents the base plate from undergoing out-of-plane deformation so as to deform such that the upper face projects out.
In the above column base structure, the column is offset and joined to the base plate, necessitating the out-of-plane deformation prevention means. The column base structure becomes more complicated as a result, leaving room for improvement.